Male Reproductive System of Animals

Male Reproductive system include primary, secondary and accessory sex organs. Primary sex organs include two testes which are normally located in an external sac of skin called the scrotum. Secondary sex organs include duct system, epididymis and the penis. The penis is transversed by urethra, which is the common passage for urine and semen. The accessory organs include prostate gland, a pair of seminal vesicles, and a pair of Cowper’s glands or bulbo-urethral glands.

Scrotum:
Scrotum is a two-lobed sac developed from the invagination of inguinal skin to accommodate the testes. Although it is apparently divided into two almost equal halves by the median vertical band, the left half is slightly longer and more voluminous than the right half.

Main function of scrotum is to support and protect the testes suspended by the spermatic cord in the scrotal sac. The scrotum functions as a heat-regulating mechanism in the male. It keeps the testicles 4-5°C below normal body temperature. This lowered temperature is essential for sperm formation. The large number of sebaceous and sweat glands on the scrotum help in lowering the scrotal temperature. During the hot season, the thermoregulatory action causes the tunica dartos muscle to relax, allowing the scrotum to elongate, dropping the testes far from the heat of the body. During the cold season, the scrotal muscle contracts, retracting the scrotum and bringing the testes nearer to the body. This thermoregulatory action, however, does not begin until the animal approaches puberty.

Testes:The testes are primary sex organs in the male. They are found in pairs suspended in the scrotal sac by the spermatic cord outside the abdominal cavity in the inguinal region. Each testis is an independent unit, separated from the other in the scrotal sac. The testes are firm and compact masses of parenchymatous tissue.

The testes develop within the abdominal region near the kidneys. They commence their descent from the abdomen into the scrotal pouches during fetal development. Migration is normally completed by the time of birth or soon after birth. Sometimes one or both of the testes may fail to descend into the scrotum during maturity. This condition is known as unilateral or bilateral cryptorchidism. Bulls affected by bilateral cryptorchidism are sterile. This condition is thought to be an inherited trait, hence such bulls are not selected for further breeding.

Testes consist of a mass of coiled tubes called seminiferous tubules which produce spermatozoa from the germinal epithelial layer by a series of cell divisions. The seminiferous tubules join to form the rete testes in the mediastinum, and these, in turn, lead into a dozen efferent ducts, the vasa efferentia, which finally converge at the dorsal part of the mediastinum to form the beginning of the epididymis.

The main functions of the testes are production of viable, potentially fertile spermatozoa, and androgens or male hormone. Main sex hormone, testosterone, is secreted by the Leydig cells or interstitial cells of the testes. The secretion of this hormone is regulated by the luteinizing hormone of the anterior pituitary gland. Testosterone is responsible for the development and maintenance of the functions of the male reproductive tract, secondary sex characteristics, and sexual behavior.

Epididymis:The epididymis emerges from the joining of the vas efferentia at the dorsal part of the testis. It is a very long single duct, highly convoluted and appearing as a mass of tubes. It is comprised of three parts: the caput epididy­mis (head), the corpus epididymis (body), and the cauda epididymis (tail). The tail of the epididymis opens into the vas deferens.

Throughout most of its length, the epididymal tube is lined with secretary cells. Spermatozoa accumulate in the epididymis and mature during their passage through it. In epididymis, spermatozoa mature and become able to move spontaneously and fertilize the ovum (egg) when they come in contact with it.

Vas Deferens (Ductus Deferens):The vas deferens is a tube emerging from the tail end of the epididymis. It starts from the base of the testes, extends upward, and in association with the spermatic cord runs through the inguinal ring, where it separates itself from the arteries, veins, nerves, and other cord tissues. (Spermetic cord consists of testicular artery, testicular vein, testicular nerve, vas deference, lymphatic and cremaster muscles). Vas deference passes through the abdominal cavity towards the pelvis and finally empties into the urethra. The lumen of the vas deferens is narrow and lined with mucous membrane. The wall is made up of longitudinal and circular layers of involuntary muscles covered by the outer layer of the peritoneum. The muscles of the vas deferens contract involuntarily during ejaculation of semen and help in the expulsion of spermatozoa. In the pelvic region, the vas deferens enlarges to form the ampulla of Henle. The ampulla has numerous glands, and spermatozoa often accumulate here before ejaculation. The glands of the ampulla secrete fructose and citric acid which provide nutrition for the spermatozoa.

Urethra:The urethra is the common passage for the excretion of urine and semen. It extends through the pelvis and the penis and ends at the tip of the glans penis as the external urethral orifice. In the urethra, spermatozoa mix with the seminal plasma of the accessory sex fluids at the time of ejaculation.

Penis:The penis is essentially composed of erectile tissue. It is divided into three portions: the attached portion is called the root, the main portion is called the body, and the free portion is called the glans penis. The erectile tissue is a sponge-like system of blood vessels which becomes filled with blood under pressure when the bull is sexually stimulated. This helps the penis to enlarge and become rigid, thus enabling it to enter into the vagina of the female.

The penis of the bull contains very little erectile tissue. The penis is a cylindrical organ with a tapering end; the tapering portion opens at the angular end of the triangular sheath. When the bull is not sexually excited, the penis is in an 'S'-shaped form known as a sigmoid flexure. During erection, the longitudinal flexure straightens, thereby increasing the length of the organ. The erector muscle pulls the penis against the pelvis and aids in erection by compressing the veins of the penis. The retractor muscles help to return the extended penis to the flexed state.

Accessory Sex Glands:The accessory sex glands of the male reproductive system are a pair of seminal vesicles, the prostate gland, and Cowper's glands or bulbo-urethral glands. They provide the bulk of the seminal plasma.

Seminal Vesicles - Each of the two seminal vesicles are located on either side of the ampulla. The seminal vesicles are lobulated and highly secretary. They open either above or below the opening of the vas deferens. The secretion of the seminal vesicles contains a large amount of fructose and citric acid, which are required for the nourishment of spermatozoa.

Prostate Gland - The body of the prostate is situated in front of the seminal vesicles on the dorsal surface of the pelvic urethra near the neck of the urinary bladder. It secretes a mineral-rich fluid.

Cowper's Glands or Bulbo-urethral Glands - The Cowper's glands are two in number and lie one on each side of the pelvic urethra, partially buried in the transverse-cavernous muscle. They produce a viscid, mucous-like lubricating substance.